// set 去重
var findSubsequences = function(nums) {
    const res = [];
    const n = nums.length;
    const set = new Set();
    const back = (path,index) => {
        if(path.length>=2) {
        const str = path.toString(); // path数组 转成字符串
        if(!set.has(str)) {         // set中没有存有当前path
        res.push(path.slice());    // 推入一份path的拷贝
        set.add(str);    
        }
                // 存入set，记录一下
      }
        for(let i = index; i < n; i++) {
           if(nums[i]>=path[path.length-1] || path.length==0) {
               path.push(nums[i])
               back(path,i+1);
               path.pop()
           }
        }
    }
    back([],0)
    return res;
};